As is well known, disk drives are used for the storage of digital data, which is provided in circular tracks on the magnetic surface of a disk. Typically, each such track contains, at predetermined radial locations therealong, an address code which identifies that track. The physical address and servo data for each track to be accessed is written during a previous servo writing operation.
Typically, each track address is expressed in Gray code, rather than conventional binary representation. In Gray code, as a number is incremented, only a single bit of the code changes, while, for example, many bits may change as a conventional binary code number is incremented, as shown in the following example:
______________________________________ decimal conventional track no. binary code Gray code ______________________________________ track 7 0111 0100 track 8 1000 1100 ______________________________________
As seen in this example, all four bits of the conventional binary code change as the code is incremented from decimal 7 to decimal 8, while in Gray code only a single bit changes when going from decimal 7 to decimal 8.
Thus, if conventional binary code were to be used, an erroneous or misdetected bit reading could give an erroneous value which could result in a serious track reading error.
As the heads move toward a chosen track to read or write data thereon, track addresses are read so that the location of the heads can be determined as they move toward the chosen track. Then, based on this information, the distance to the target track and velocity of the heads are computed.
Gray code information is used by the servo system in two modes; seek mode and track following mode. In track following mode the disk drive may read or write data to a specific target track. During this mode the head is positioned over the center line of the target track. The track address is decoded and used to verify that the heads are positioned on the correct track. Since no address bits are expected to change, any binary code sufficient in length to define the number of tracks unambiguously is satisfactory. For example, 12 bits of information in Gray code would allow 4096 track addresses.
During the seek mode, heads are moving rapidly in a radial direction from a first track to a target or destination track in order to access data on another portion of the disk. It is desirable to accomplish this task as quickly as possible to avoid unnecessary delays in providing data to the host computer. In seek mode a track address is read to determine head location, and is used to compute the distance to the target track as well as radial velocity of the heads. The accuracy and time of the seek operation depends on the accuracy with which instantaneous track address information can be decoded.
Errors in determining velocity and/or position affect the overall time to data because in such case the actuator velocity will not be optimum, i.e., seek velocity will either be higher or lower than optimum. If velocity is lower than optimum then the seek operation will take longer than needed. If such a state happens randomly, seek specifications will need to be changed to reflect occasional slower seeks. If velocity is higher than optimum, especially near the end of a seek operation, the actuator may over shoot the track with highly undesirable effects.
As the heads are moving toward the target track, they may be positioned over a boundary of two tracks during the period when the track address is being read. In this case the heads may read bits from one track address or the other in any combination. Had the addresses been encoded in conventional binary form, serious decoding errors could occur as shown in the following example:
______________________________________ conventional binary code decimal ______________________________________ bit number 7 6 5 4 3 2 1 0 track 31 0 0 0 1 1 1 1 1 31 track 32 0 0 1 0 0 0 0 0 32 detected address 0 0 0 0 0 1 0 1 05 value track error 26 ______________________________________
In this example, bits 7, 6, 5, 2 and 0 (underlined above) are read from track 31, while bits 4, 3 and 1 (underlined above) are read from track 32, as shown in "detected address value". When decoded the address will be read as track 5, an error of 26 tracks.
In comparison, using the same read bit pattern with Gray code, an error of only one track would have occurred:
______________________________________ Gray code decimal ______________________________________ bit number 7 6 5 4 3 2 1 0 track 31 0 0 0 1 0 0 0 0 31 track 32 0 0 1 1 0 0 0 0 32 read address 0 0 0 1 0 0 0 0 31 track error 1 ______________________________________
Based on the assumption that data from no more than two track addresses will be read, servo systems have been developed using supplementary servo patterns which can be used to detect and correct track position when a track is found to be at an unexpected odd or even position error condition. Information is derived from the patterns indicating the magnitude and direction of the error.
The assumption that an error will not exceed a single track is valid as long as the heads do not cross more than one track boundary while an address is being read. Operationally, this assumption has been practical for previous disk drives. However, modern high performance disk drives have tracks that are much closer together than in the past, and much faster track seeking servo systems are used. Furthermore, the increased number of tracks on a disk surface has tended to increase the number of bits of Gray code required to be read in each track information burst, which reading normally takes place at fixed intervals of time. The combination of these factors has created disk drives which, during seek, may cause the heads to cross more than one track boundary so that partial Gray code addresses from several tracks may be read.
When multiple tracks are crossed during address reading, a complete Gray code of a track cannot be read because of the very fast movement of the head toward the target track. Citements from several tracks would be combined to obtain a complete "detected address value" which may have more than a single bit in error. In such a case, the fact that one of the most significant bits had changed during the accessing operation would not have been detected because the read heads might have already advanced past the location where the bit changed, and the least significant bit pattern would be misinterpreted to imply an incorrect track address. Based on this incorrect reading, recovery correction necessary to appropriately reach the target track would not be achieved.
In explaining this, reference is made to Table 1 below, wherein decimal track numbers and their corresponding Gray codes are set forth, with each Gray code arranged from left to right from most significant bits to least significant bits.
As shown in the flow chart of FIG. 1, the process starts in block 11. The GRAY code address and the appended bit are read in block 12 and then converted to a binary track address in block 13. If the appended bit from block 13 is the expected one, as determined by comparison block 14, the operation is ended at block 16.
If the comparison results is negative, indicating that the appended bit was not as expected, block 17 is entered to determine the proper track address, block 18 is entered and the corrected address is calculated using the algorithm shown, and the operation ends at block 16.
If the operation in block 17 indicates an outward seek is required, the operation enters block 19 to calculate the correct track address using the algorithm shown, and the operation ends.
TABLE I ______________________________________ Gray Code MSB LSB Decimal Track No. .rarw. .fwdarw. ______________________________________ 500 0100001110 501 0100001111 502 0100001101 503 0100001100 504 0100000100 505 0100000101 506 0100000111 507 0100000110 508 0100000010 509 0100000011 510 0100000001 511 0100000000 512 1100000000 513 1100000001 514 1100000011 515 1100000010 516 1100000110 517 1100000111 518 1100000101 519 1100000100 520 1100001100 521 1100001101 522 1100001111 523 1100001110 ______________________________________
During a seek in operation, the heads may at random read several significant bits from decimal track 511, for example 010, and may read bits from tracks 512, 513, 514 and/or 515, all zeros. Then, the head may read the least significant bits 110 of the track 516. The Gray code thus read is 0100000110, indicating decimal track 507, when the heads are actually positioned at track 516, having read the last bits of the address (110) therefrom, an error of 9 tracks.
Errors of this type have serious consequences for the servo system computation resulting in inaccurate velocity and trajectory calculations in attempting to reach the target track. In some cases a seek failure could occur requiring additional time-consuming corrective operations to be initiated.